Conventionally, lead or lead-alloy electrodes have mainly been used as the anode for chromium plating. Although the lead or lead-alloy electrodes satisfactorily function to oxidize trivalent chromium ions formed on the cathode to hexavalent chromic acid, its chemical and electrochemical corrosion resistance is so poor that lead dissolves into the chromium-plating bath to form insoluble lead chromate or lead sulfate, which accumulates as sludge in the plating tank. In order to remove the sludge, the plating operation is suspended.
As a substitute for the lead or lead-alloy electrode, an anode comprising a substrate made of a valve metal such as titanium, and, formed on the substrate, a covering layer containing a platinum group metal or an oxide thereof, is coming to be used.
However, such an electrode obtained by covering a substrate made of titanium or other valve metal with a layer containing a platinum group metal or an oxide thereof has disadvantages as described below, although use of this electrode as an anode for chromium plating is free from the sludge formation accompanying the use of lead or lead-alloy electrodes. Disadvantages include that the above electrode is costly, and, further more, there is a problem that since the electrode is insufficient in ability to anodize trivalent chromium ions resulting from reduction of chromic acid on the cathode during plating into hexavalent chromic acid on the anode, the concentration of trivalent chromium ions in the plating bath increases, and, as a result, the plating bath weakens chromium deposit-covering power. Also, there are cases where sufficiently glossy deposits cannot be obtained. In addition, the electrical conductivity of the plating bath decreases, making it difficult to conduct the chromium plating normally.
Recently, chromium-plating baths containing various kinds of additives of organic materials, such as sulfonic acid-based baths, have been developed as substitutes for the conventional Sargent bath and hydrosilicofluoric acid baths having considerable corrosive properties, and have come into common use. Compared to the conventional Sargent bath, the chromium-plating bath containing an additive of an organic material has attained a higher cathode current efficiency and improved plating efficiency, and also has an advantage that chromium-plated products produced using this plating bath have improved quality.
Such a chromium-plating bath containing an additive of an organic material, however, has a problem in that if a lead or lead-alloy electrode is used as an anode in this plating bath, the electrode is consumed more rapidly than the same electrode in the conventional Sargent bath; hence, such use involves a problem.
A platinum-plated electrode obtained by covering a substrate made of a valve metal such as titanium with platinum by electroplating is also being used as an anode in plating baths, as an alternative to the lead or lead-alloy electrode. Although this platinum-plated electrode has a high electrode potential and is excellent in the ability to anodize trivalent chromium ions formed by cathodic reduction into hexavalent chromic acid on the anode, it is defective in that since the chromium-plating bath contains an organic material, the platinum is consumed at a high rate, and, hence, the thickness of the platinum deposit covering the substrate should be increased in order to maintain long-term and stable chromium plating. This raises the cost of the electrode. Therefore, the cost advantage brought about by the replacement of the conventional Sargent bath with the chromium-plating baths containing organic ingredients is diminished.
On the other hand, as an expedient for improving the corrosion resistance of an electrode obtained by covering a substrate made of a valve metal or an alloy thereof with an electrode catalyst coating containing a platinum group metal or an oxide thereof, provision of an intermediate layer made of a composite oxide of stannic oxide and antimony oxide between the electrode catalyst coating and the electrode substrate is disclosed, for example, in JP-B-59-2753 and JP-B-61-36075 (the term "JP-B" as used herein means an "examined Japanese patent publication"). However, any of such intermediate layers is unable to be stably present in chromium-plating baths and dissolves away within a short time period. For this reason, the above intermediate layer is ineffective in preventing the deterioration of the substrate and cannot retain its adhesion to the electrode catalyst coating layer containing a platinum group metal or an oxide thereof and, as a result, the voltage increases in a short period of time.
In addition, the electrodes having a catalyst coating comprising a platinum group metal or an oxide thereof, for example, the electrode as described in JP-B-59-2753 which has a ruthenium oxide coating, have been unable to stand practical use because they show poor corrosion resistance when used as an anode for plating, and, furthermore, their ability to oxidize trivalent chromium ions into hexavalent chromic acid is poor.
JP-B-62-2038 discloses an electrode which comprises a substrate made of a valve metal or an alloy thereof, having formed thereon an electrode catalyst coating containing a mixture of a platinum group metal and tin dioxide, and the consumption of which is reduced due to such a coating. This electrode, however, is unsuitable for chromium plating because when chromium plating is conducted using this electrode as the anode, oxygen evolved during the electrolysis increases the voltage in a short time period, making the electrode unusable any more.